Fiber optic transmission of data signals across large distances is presently accomplished using a variety of laser transmitters, which generally operate at near-infrared and infrared (IR) wavelengths.
1.3 μm in-plane (edge-emitting) lasers or vertical cavity surface emitting lasers (VCSELs) operating at a modulation bandwidth of 10 Gb/s, for the metro application using single-mode fiber, will allow data transmission up to a distance of 20–30 km. J. S. Harris, Jr., “Tunable Long-Wavelength Vertical-Cavity Lasers: The Engine of Next Generation Optical Networks?” IEEE J. Select. Topics Quantum Electron., Vol. 6, No. 6, November/December 2000, pp. 1145–1160. Ideally, such devices should operate with high performance up to high temperatures (85° C.) in an uncooled environment. Long-haul fiber optic transmission requires the use of 1.55 μm sources, encouraging the development of low-cost 1.55 μm VCSELs. However, conventional InP-based long wavelength diode lasers, at λ=1.3–1.55 μm, are inherently highly temperature sensitive. As a result, additional electronics are needed to maintain the operational temperature of the lasers. This additional temperature controller leads to a significant increase in the laser packaging cost.
Longer wavelength, λ>1.5 μm, (eye safe) high power sources are also needed for laser-based radar and free-space optical links. To minimize atmospheric disturbances, these applications prefer sources in the mid-IR (2–5 μm) wavelength region. Efficient, room temperature operating mid-IR sources are also needed for the development of compact chemical sensing systems, and also for infrared countermeasures against heat-seeking missiles that threaten both military and commercial airliners. However, the strong temperature sensitivity and radiative inefficiency of conventional long-wavelength InP-, InAs-, and GaSb-based lasers severely impacts their CW (continuous wave) performance. While Quantum Cascade (QC) lasers are available for wavelengths greater than 5 μm, no diode laser sources are currently available which operate CW at room temperature in the 3–5 μm wavelength region. It would be desirable to be able to produce efficient mid-IR (2–5 μm) sources on conventional GaAs or InP substrates as compared to existing type-I and type-II quantum well (QW) lasers which utilize GaSb or InAs substrates.
The strong temperature sensitivity of conventional type-I quantum well long wavelength diode lasers stems from various physical mechanisms, including Auger recombination, large carrier leakage from the active layer, intervalence band absorption, and a strongly temperature dependent material gain parameter. A. F. Phillips, A. F. Sweeney, A. R. Adams, and P. J. A. Thijs, “Temperature Dependence of 1.3- and 1.5-μm Compressively Strained InGaAs(P) MQW Semiconductor Lasers,” IEEE. J. Select. Topics Quantum Electron., Vol. 5, No. 3, May/June 1999, pp. 401–412. Typical values for the conduction-band offset for 1.3 μm InGaAsP-active region on InP-substrate lasers are ≈200 meV, which is too low to prevent severe carrier leakage from the quantum wells as well as increased Auger-assisted carrier leakage. Contrary to InP-based lasers, very large conduction band offset (470 meV) can be achieved for a 1.3 μm emitting quantum well active region with GaAs confinement layers. The large band offset helps to suppress thermally-induced carrier leakage out of the quantum well to the confining region and also the Auger-assisted carrier leakage. See J. S. Harris, Jr., “Tunable Long-Wavelength Vertical-Cavity Lasers: The Engine of Next Generation Optical Networks?” IEEE J. Select. Topics Quantum Electron., Vol. 6, No. 6, November/December 2000, pp 1145–1160 and H. C. Casey, Jr., “Temperature dependence of threshold current density on InP-Ga0.28In0.72As0.6P0.4(1.3 μm) double heterostructure lasers,” J. Appl. Phys., Vol. 56 (7), 1984, pp. 1959–1964.
Due to the potential advantages offered from 1.3–1.55 μm GaAs-based lasers, there have been many efforts directed towards achieving 1.3–1.55 μm emission using various types of active material on GaAs substrates. In(Ga)As quantum dots have been pursued by many different groups with various degrees of success. Quantum dot active lasers exhibit extremely low transparency current densities because of the small active volume. However, low modal gain, high temperature sensitivity, and poor modulation response are still issues under development by many groups. Other promising alternatives for 1.3–1.55 μm emission active regions include the use of GaAsSb-active materials.
An attractive approach for achieving long-wavelength laser emission on GaAs substrates is the use of highly-strained InGaAs or InGaAsN QWs. The use of highly-strained InGaAs QW active lasers to extend the emission wavelength to 1.20 μm was reported in S. Sato and S. Satoh, “1.21 μm Continuous-Wave Operation of Highly Strained GaInAs Quantum Well Lasers on GaAs Substrates,” Jpn. J. Appl. Phys., Vol. 38, 1999, pp. L990–L992; F. Koyama, D. Schlenker, T. Miyamoto, Z. Chen, A. Matsutani, T. Sakaguchi, and K. Iga, “1.2 μm highly strained GalnAs/GaAs quantum well lasers for singlemode fibre datalink,” Electron. Lett., 35(13), 1999, pp. 1079–1081; D. Schlenker, T. Miyamoto, Z. Chen, F. Koyama, and K. Iga, “1.17-μm highly strained GaInAs-GaAs quantum-well laser,” IEEE Photon. Technol. Lett., Vol. 11 (8), August 1999, pp. 946–948. Recently, GaAsP tensile-barriers have also been implemented to strain-compensate the InGaAs quantum wells. The reduction in the bandgap of the InGaAsN materials, reported in M. Kondow, T. Kitatani, S. Nakatsuka, M. C. Larson, K. Nakahara, Y. Yazawa, M. Okai, and K. Uomi, “GaInNAs: A novel material for long wavelength semiconductor lasers,” IEEE J. Select. Topic Quantum Electronic., Vol. 3, 1997, pp. 719–730, due to the presence of the N, is also followed by reduction in the compressive strain of the material due to the smaller native lattice constant of InGaN compound. Since then, many promising results have been demonstrated for 1.3 μm InGaAsN-active lasers.
Some of the highest performance InGaAsN-based lasers to date have been grown by molecular beam epitaxy (MBE). However, for manufacturing considerations such as high-throughput, the use of metal organic chemical vapor deposition (MOCVD) growth is preferable. The optimization of MOCVD grown InGaAsN materials and an understanding of growth limitations are essential to achieve this goal.
Nitrogen is usually incorporated into the InGaAs-quantum well using low temperature MOCVD growth with dimethylhydrazine as the nitrogen source. Early studies of InGaAsN-active lasers were disappointing in that nitrogen concentrations of 2–3% resulted in poor room temperature PL intensity and very high threshold current density lasers. S. Sato, “Low Threshold and High Characteristics Temperature 1.3 μm Range GaInNAs Lasers Grown by Metalorganic Chemical Vapor Deposition,” Jpn. J. Appl. Phys., Vol. 39, June 2000, pp. 3403–3405. Only recently, lower MOCVD growth temperatures have been utilized to achieve higher-indium incorporation, without strain relaxation, thereby requiring smaller amounts of nitrogen to achieve 1.3 μm-emission. T. Takeuchi, Y. -L. Chang, M. Leary, A. Tandon, H. -C. Luan, D. P. Bour, S. W. Corzine, R. Twist, and M. R. Tan, “Low Threshold 1.3 μm InGaAsN Vertical Cavity Surface Emitting Lasers Grown by Metalorganic Chemical Vapor Deposition,” IEEE LEOS 2001 Post-Deadline Session, San Diego, USA, November 2001; N. Tansu and L. J. Mawst, “Low-Threshold Strain-Compensated InGaAs(N) (λ=1.19–1.31 μm) Quantum Well Lasers,” IEEE Photon. Technol. Lett., Vol. 14(4), April 2002, pp. 444–446; N. Tansu, N. J. Kirsch, and L. J. Mawst, “Low-Threshold-Current-Density 1300-nm Dilute-Nitride Quantum Well Lasers,” Appl. Phys. Lett, Vol. 81 (14), September 2002, pp. 2523–2525; N. Tansu, A. Quandt, M. Kanskar, W. Mulhearn, and L. J. Mawst, “High-Performance and High-Temperature Continuous-Wave-Operation 1300-nm InGaAsN Quantum Well Lasers by Organometallic Vapor Phase Epitaxy,” Appi. Phys. Lett., Vol. 83(1), July 2003, pp. 18–20; M. Kawaguchi, T. Miyamoto, E. Gouardes, D. Schlenker, T. Kondo, F. Koyama, and K. Iga, “Lasing characteristics of low threshold GaInNAs lasers grown by Metalorganic Chemical vapor Deposition”, Jpn. J. Appl. Phys., Vol. 40, July 2001, pp. L744–L746. The smaller nitrogen content (<0.5%) in the InGaAsN-active region results in significantly improved PL intensity and reduced threshold current densities (0.225 kA/cm2). N. Tansu, N. J. Kirsch, and L. J. Mawst, “Low-Threshold-Current-Density 1300-nm Dilute-Nitride Quantum Well Lasers,” Appl. Phys. Lett, Vol. 81 (14), September 2002, pp. 2523–2525.
One of the challenges in growing InGaAsN QW lasers by MOCVD is the difficulty of incorporating N into the InGaAs QW, while maintaining a high optical quality film. The low purity of the N-precursor used in MOCVD (U-DMHy) is also suspected as a possible reason for the low optical quality of MOCVD-grown InGaAsN QWs. In order to incorporate sufficient N into the InGaAsN QW, very large [DMHy]/V (as high as 0.961) is required. Due to the high cost and the low purity of the DMHy precursor, lowering the [AsH3]/III to achieve large [DMHy]/V would be the preferable option to increasing the DMHy flow. As a result, a large [DMHy]/V ratio requires the [AsH3]/III ratio to be rather low. T. Takeuchi, Y. -L. Chang, A. Tandon, D. Bour, S. Corzine, R. Twist, M. Tan, and H. -C. Luan, “Low threshold 1.2 μm InGaAs quantum well lasers grown under low As/III ratio,” Appl. Phys. Lett., Vol. 80(14), April 2002, pp. 2445–2447 has demonstrated that the growth of an InGaAs QW (λ=1200 nm) with the very low [AsH3]/III ratio is significantly more challenging compared to the case in which tertiary butyl arsine (TBA) is utilized as the As-precursor. As the [AsH3]/III ratio is reduced, the luminescence of the InGaAs QW reduces rapidly for low [AsH3]/III (below 15–20), which is, however, required for achieving sufficiently large [DMHy]/V. These challenges have resulted in difficulties in realizing high performance MOCVD-lnGaAsN QW lasers with AsH3 as the As-precursor until recently. See N. Tansu, and L. J. Mawst, “Low-Threshold Strain-Compensated InGaAs(N) (λ=1.19–1.31 μm) Quantum Well Lasers,” IEEE Photon. Technol. Lett., Vol. 14(4), April 2002, pp. 444–446; N. Tansu, N. J. Kirsch, and L. J. Mawst, “Low-Threshold-Current-Density 1300-nm Dilute-Nitride Quantum Well Lasers,” Appl. Phys. Lett, Vol. 81 (14) September 2002, pp. 2523–2525; N. Tansu, A. Quandt, M. Kanskar, W. Mulhearn, and L. J. Mawst, “High-Performance and High-Temperature Continuous-Wave-Operation 1300-nm InGaAsN Quantum Well Lasers by Organometallic Vapor Phase Epitaxy,” Appl. Phys. Lett., Vol. 83(1), July 2003, pp. 18–20. In this approach, the design of the active region is based on strain-compensated InGaAsN QW, with very high In content (In˜40%) and minimum N content (N˜0.5%), to achieve 1300-nm emission. Minimum N content in the InGaAsN QW allows the growth of the active region with an optimized AsH3/III ratio. Through growth optimization, the highest performance InGaAsN lasers reported to date have been obtained for lasers emitting up to 1.38 μm. N. Tansu, J. Y. Yeh, and L. J. Mawst, “Low-Threshold 1382-nm InGaAsN Quantum-Well Lasers with Metalorganic Chemical Vapor Deposition,” Appl. Phys. Lett. (submitted).
While high-performance 1300-nm QW lasers have now been demonstrated by both MBE and MOCVD, a decrease in the threshold current density (Jth) of the InGaAsN QW laser is typically accompanied with a decrease in the T0 value. There are several possible factors underlying the lower T0 values of high-performance 1300-nm InGaAsN QW lasers. Previous work by R. Fehse, S. Tomic, A. R. Adams, S. J. Sweeney, E. P. O'Reilly, A. Andreev, H. Riechert, IEEE Select. J. Quantum Electron., 8(4), 801 (2002) without taking into account any carrier leakage, have attributed Auger recombination as the sole factor that leads to the lower T0 values of the high-performance InGaAsN QW lasers. However, since these studies do not account for the possibility of carrier leakage, the Auger recombination coefficients can be overestimated. Recent studies have suggested carrier leakage as well as a temperature sensitive material gain in InGaAsN QW lasers as major contributing factors leading to the lower T0 values of InGaAsN QW lasers, compared with “nitrogen-free” 1.2 μm InGaAs QW lasers. N. Tansu and L. J. Mawst, “Temperature Sensitivity of 1300-nm InGaAsN Quantum-Well Lasers,” IEEE Photon. Technol. Lett., Vol. 14(8), August 2002, pp. 1052–1054; N. Tansu and L. J. Mawst, “The Role of Hole-Leakage in 1300-nm InGaAsN Quantum Well Lasers,” Appl. Phys. Lett., Vol. 82(10), March 2003, pp. 1500–1502; N. Tansu, J. Y. Yeh, and L. J. Mawst, “Experimental Evidence of Carrier Leakage in InGaAsN Quantum Well Lasers,” Appl. Phys. Lett., Vol. 83(11), September 2003.
These processes controlling the temperature sensitivity will become of increasing significance as the emission wavelength of the InGaAsN is extended beyond λ>1.3 μm. Recent results on higher N content InGaAsN lasers with emission wavelengths of 1.38 μm, indicate that the temperature sensitivity increases as the wavelength becomes longer. Extending the emission wavelength of InGaAsN-active lasers to 1.55 μm and beyond thus remains a considerable challenge, requiring new active layer materials or new structure designs. There have been several efforts in extending the wavelength on GaAs by utilizing highly strained InGaAsN or InGaAsN(Sb) QWs. M. O. Fischer, M. Reinhardt, A. Forchel, “Room-temperature operation of GalnAsN-GaAs laser diodes in the 1.5-μm range,” IEEE J. Select. Topic Quantum Electronic., Vol. 7 (2), March-April 2001, pp. 149–151; V. Gambin, W. Ha, M. A. Wistey, S. Bank, S. Kim, and J. S. Harris “GalnNAsSb for 1.3–1.6 μm long wavelength lasers grown by MBE,” IEEE J. Quantum. Electron, Vol. 8, 2002, pp. 795–800 or InGaAs-GaAsSb type-II QWs. P. Dowd, W. Braun, D. J. Smith, C. M. Ryu, C. -Z. Guo, S. L. Chen, U. Koelle, S. R. Johnson, and Y. -H. Zhang, “Long wavelength (1.3 and 1.5 μm) photoluminescence from InGaAs/GaPAsSb quantum wells grown on GaAs,” Appl. Phys. Lett., 75 (9), 1999, pp. 1267–1269. While initial results appear promising, poorer performance, compared with conventional InP-based lasers, remains an issue. The use of InGaAsN(Sb) has allowed emission wavelengths out to 1.49 μm, although those devices exhibited very high threshold current density (16 KA/cm2). V. Gambin, W. Ha, M. A. Wistey, S. Bank, S. Kim, and J. S. Harris “GalnNAsSb for 1.3–1.6 μm long wavelength lasers grown by MBE,” IEEE J. Quantum. Electron, Vol. 8, 2002, pp. 795–800. Furthermore, extending the emission wavelengths beyond 1.5 μm with this technology is unlikely due to the high strain of the quantum well employed. New dilute-nitride-based active layer materials are needed to enable device performance surpassing conventional InP- and GaSb-based lasers with long wavelength emission.